Implantable prostheses are commonly used in medical applications. One of the more common prosthetic structures is a tubular prosthesis which may be used as a vascular graft to replace or repair a damaged or diseased blood vessel. In particular, tubular prostheses such as vascular grafts are commonly used to repair or replace damaged or diseased vessels including damaged or diseased vessels occurring at non-uniform sites such as bifurcation points.
A bifurcation point is generally where a single lumen or artery (often called the trunk) splits into two lumens or arteries (often called branches) such as in a “Y” configuration. For example, one such bifurcation point is found within the human body at the location where the abdominal aortic artery branches into the left and right (or ipsalateral and contralateral) iliac arteries. Treatment of a bifurcation point afflicted with such defects as an occlusion, stenosis, or aneurysm is a particularly demanding application for grafts, stents or stent-grafts. Bifurcation points are exposed to high mechanical stresses based on the hemodynamics of flow at such sites. Specifically, high turbulence of blood flow is caused, for example, by the change in direction of flow as well as diminished vessel size beyond the bifurcation point. The bifurcation point of any graft or stent-graft must therefore be able to withstand high levels of mechanical stress.
It is well-known to form a tubular graft from polymers such as polytetrafluoroethylene (PTFE). Moreover, as is well-known, a tubular graft may be formed by stretching and expanding PTFE into a structure referred to as expanded polytetrafluoroethylene (ePTFE). Expanded PTFE consists of a unique microstructure of nodes interconnected by fibrils.
It is particularly desirable to make implantable prostheses from ePTFE as ePTFE exhibits many desirable characteristics. In particular, ePTFE exhibits the desirable characteristics of superior biocompatibility and low thrombogenicity. Moreover, tubes of ePTFE may be formed to be exceptionally thin yet exhibit the requisite strength necessary to serve in the repair or replacement of a body lumen. The thinness of ePTFE tubes facilitates ease of implantation and deployment with minimal adverse impact on the body.
Moreover, ePTFE in many ways satisfies a goal in graft technology to mimic, as closely as possible, the natural function of the blood vessel being replaced. Expanded PTFE is strong enough to resist tear and other mechanical damage under normal conditions, sufficiently flexible and compliant to accommodate the natural variability of blood flow and pressure, and sufficiently porous to allow for enhanced healing and appropriate tissue ingrowth when it is desired to anchor a prosthesis made therefrom within a blood vessel of the body.
Additionally, as the process of expanding PTFE into a cylindrical shape can result in a tubular graft having uniform or substantially uniform node and fibril size and orientation, tubular grafts made from ePTFE, as a general matter, are particularly strong due to the underlying uniformity of the ePTFE microstructure.
Attempts to form bifurcated or other complicated shaped structures by expanding PTFE into non-uniform or complicated shapes, however, have met with difficulties. For example, stretching and expanding PTFE into a Y-shaped, bifurcated graft is difficult and may result in non-uniform node and fibril size and orientation. A graft so formed may disadvantageously have properties which are not uniform throughout the graft as a result. Furthermore, when Y-shaped grafts are stretched, the nodes and fibrils of the least uniform size and orientation are likely to be predominantly located at the bifurcation point of the Y-shaped graft. These attributes make the non-uniform section (e.g., the bifurcation point) of the graft less strong than the more uniformly sized and oriented sections of the graft. As discussed above, however, it is the bifurcation point where high levels of mechanical stress can exist.
Methods for making bifurcated ePTFE grafts to date include forming multiple tubular sections of ePTFE and attaching them to one another to form a bifurcated graft. For example, it is possible to form a trunk and branches separately and then combine them to form an integral bifurcated graft. Such methods involve, for example, forming a seam at the juncture between the trunk and branch sections by lamination, co-melting, stitching or other attachment methods known in the art. Formation of such a seam at a bifurcation point may be disadvantageous because that section of the graft may see heightened mechanical stress when placed in the body.
Accordingly, there is a need for new methods of making a bifurcated implantable prosthesis which overcome the disadvantages of the prior art.